Refractive errors

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Last updated 10:34 AM on 9/18/26
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4 Terms

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Emmetropia & Basics




  • Parallel rays focus precisely on the retina with accommodation at rest; axial length is normally 24mm and total dioptric power is +60D. 1 mm of change in axial length equals a 3D refractive error shift.


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Myopia (Short-sightedness)

  • Parallel rays focus in front of the retina. Corrected with concave (minus) lenses, PRK, LASIK, or phakic IOLs (ICL)..



  • Pathological Myopia: Progressive elongation with degenerative fundus changes including myopic crescent/conus, chorioretinal atrophy, posterior staphyloma, retinal holes, and retinal detachment. Low-dose atropine 0.01% is used medically to slow progression in children


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Hypermetropia (Long-sightedness)

  • Parallel rays focus behind the retina. Corrected with convex (plus) lenses.



  • Associated with asthenopic symptoms (frontal headaches during near work), shallow anterior chambers, and an increased risk of accommodative convergent squint and angle-closure glaucoma.


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Astigmatism

  • Condition where light rays fail to form a single point focus due to unequal meridian refraction.


  • Regular Astigmatism: Principal meridians are at right angles. With-the-rule (vertical meridian steeper due to eyelid pressure) vs. Against-the-rule.

  • Irregular Astigmatism: Principal meridians are not at right angles (commonly caused by keratoconus or corneal scarring; treated with rigid contact lenses or surgery).